Screencap of the Final Boss Battle.
The Accurst Queen of Karst serves as Momodora: Reverie Under the Moonlight's Underworld Queen and final boss.
She's responsible for the spread of darkness and corruption that's overtaken her country and threatens the rest of the world. Her actions have also caused the despair and misery of her subjects, and forced the dead to roam the land once again.
Kaho Reinol arrives at Karst with the intent of stopping her and the spread of evil. By the end, she succeeds in purifying the cursed queen at the cost of her own life.
It's implied the queen may have been forced into evil by demonic possession, similar to Isadora's stint as the Underworld Queen.